Fractionation of eutectic mixtures of dimethylnapthalenes by dianhydride complexation

ABSTRACT

Binary eutectics of dimethylnapthalenes having each methyl group at a Beta position cannot be fractionated by crystallization and are difficult to fractionate by distillation. However, by dissolving the normally solid eutectic in an inert solvent and contacting the resulting solution with a dianhydride of 1,2,4,5,benzenetetracarboxylic acid or a dianhydride of 1,2,3,4benzenetetracarboxylic acid, a solid complex is formed. Separation of the solid complex and its subsequent decomposition results in a complexate that is substantially richer in a dimethylnapthalene which is preferentially complexed. Dimethylnaphthalenes are oxidized to naphthalenecarboxylic acids which are used in the production of dyes and pigments.

United States Patent Davis et al.

[451 May 23, 1972 [54] FRACTIONATION OF EUTECTIC MIXTURES OFDIMETHYLNAPTHALENES BY DIANHYDRIDE COIVIPLEXATION [72] Inventors:Ronald 1. Davis, Wilmington, Del.; Kenneth A. Scott, Swarthmore, Pa.

[73] Assignee: Sun Oil Company, Philadelphia, Pa.

[22] Filed: May 1, 1970 [21] Appl. No.: 33,980

[ 52] U.S. Cl. ..260/674 N [51] lnt.Cl.... ..C07c 7/00 [58] Field ofSearch ..260/674 N [56] References Cited UNITED STATES PATENTS 2,885,4535/1959 Fishel ..260/674 2,941,017 6/l960 Veatch et al ....260/6742,914,581 1 H1959 Christensen et al. ..260/674 OTHER PUBLICATIONSNaletova et al., Chemical Abstract, Vol. 68, Abstract No. 24782m 1967.

Primary ExaminerDelbert E. Gantz Assistant Examiner-C. E. SpresserAtt0rney-George L. Church, Donald R. Johnson and Wilmer E. McCorquodale,Jr.

57 ABSTRACT Binary eutectics of dimethylnapthalenes having each methylgroup at a B position cannot be fractionated by crystallization and aredifficult to fractionate by distillation. However, by dis solving thenormally solid eutectic in an inert solvent and contacting the resultingsolution with a dianhydride of l,2,4,5, benzenetetracarboxylic acid or adianhydride of 1,2,14- benzenetetracarboxylic acid, a solid complex isformed. Separation of the solid complex and its subsequent decompositionresults in a complexate that is substantially richer in adimethylnapthalene which is preferentially complexed.Dimethylnaphthalenes are oxidized to naphthalenecarboxylic acids whichare used in the production of dyes and pigments.

9 Claims, N0 Drawings FRACTIONATION OF EUTECTIC MIXTURES OFDIMETHYLNAPTHALENES BY DIANHYDRIDE COMPLEXATION CROSS REFERENCES TORELATED APPLICATIONS The present application is copending with thefollowing listed applications filed of even date herewith, allapplications being of common ownership.

Ser. No.

Inventor(s) Title 33.949 R. l. Davis K. A. Scott 33,950 R. I. DavisBACKGROUND OF THE INVENTION This invention relates to a process forfractionating binary eutectics of dimethylnaphthalenes having eachmethyl group at a B position. Said eutectic mixtures consist essentiallyof 2,6- and 2,7-dimethylnaphthalenes or 2,3- and2,6-dimethylnaphthalenes or 2,3- and 2,7-dimethylnaphthalenes.

Dimethylnaphthalenes are oxidized to naphthalenecarboxylic acids whichare used in the production of dyes and pigments. A more detaileddiscussion of the utility of dimethylnaphthalenes appears inNaphthalenecarboxylic Acids by K. A. Scott in Kirk-Othmer, ENCYCLOPEDIAOF CHEMI- CAL TECHNOLOGY, 2nd Edition, Vol. 13.

For convenience dimethylnaphthalene or dimethylnaphthalenes herein willbe referred to as DMN with specific DMN isomers being indicated byreference to the location of the methyl groups. For example,2,6-dimethylnaphthalene will be referred to as 2,6-DMN.

DMN are found in coal tar, lignite tar, crude oil, the drip-oil fractionproduced during the pyrolysis of hydrocarbons to make olefins, in heavypetroleum reformate and in petroleum gas oil produced by catalyticcracking. In these hydrocarbon mixtures DMN are usually present inrather dilute concentration. For example, one analysis shows DMN makingup about 4 percent by weight of a gas oil. However, by known processessuch as distillation, crystallization and solvent extraction, DMN can berecovered in concentrated form from the previously mentioned sources.

However, by the aforementioned known processes, the resultingconcentrated DMN can be a binary eutectic of DMN, i.e., 2,6- and 2,7 DMNor 2,3- and 2,6-DMN or 2,3- and 2,7-DMN. The weight or moleconcentration of binary eutectics, along with their freezing points, areshown in the following Table I.

TABLE I Eutectics of DMN* Weight or Mole Percent 2,6DMN 2,7-DMN 2,3-DMNEutectic Freezing Point, "F.

Bulletin De LAcademic Polonaise Des Sciences Vol. IX. No. l l, l96l;0nSolid- Liquid Equilibria between CoaLTar Constituents. T. Gruberski.

Thus, for example, since both 2,6-DMN and 2,7-DMN have a boiling pointof 504 F., further fractionation of this DMN eutectic by eithercrystallization or distillation is impossible. The DMN in the othereutectics have sufficiently close boiling points to make distillationdifficult.

Yet in oxidizing these DMN compounds to naphthalenecarboxylic acids, itis preferable that each isomer be oxidized by itself since each onerequires somewhat different reaction conditions for optimum oxidation.Also, a single acid may be wanted. Thus, there is a need for a methodfor separating a DMN eutectic into its isomers.

SUMMARY OF THE INVENTION This invention relates to a method for thefractionation of a binary eutectic of DMN by selective complexing withthe dianhydride of l,2,4,5-benzenetetracarboxylic acid or the dianhydride of l,2,3,4-benzenetetracarboxylic acid. The normally solideutectic is dissolved in an inert solvent and contacted with either ofthese dianhydrides. The resulting solid DMN-dianhydride complex isseparated from the liquid solution. The solid complex is decomposed andthe subsequently released DMN has a composition substantially differentfrom the original eutectic.

DESCRIPTION The complexing agent used in this invention is thedianhydride of l,2,4,S-benzenetetracarboxylic acid, also known aspyromellitic dianhydride or dianhydride of l,2,3,4-benzenetetracarboxylic acid. These two dianhydrides for convenience arereferred to herein as PMDA (I) and DA (II), respectively. The structuresrepresenting these two dianhydrides are as follows:

(I) (II) Surprisingly, the trianhydride ofl,2,3,4,5,6-benzenehexacarboxylic acid will not complex with theeutectic of 2,6- DMN and 2,7-DMN.

It is believed that the complexes formed herein are 11' -complexes,i.e., they are caused by combination between the 11- electrons of thetwo rings involved. The dianhydride apparently accepts a share in the 1relectrons of the compound which is complexed with it. Steric factorsappear to have a strong effect, since according to the theory of 1:complex formation, the two rings must be close together and parallel inorder for the complex to form. These complexes are distinct from theacid-base type as exemplified by complexes of I-IF'BF and xylenes andalso from clathrate complexes of, for example, the urea-paraffin type.

The amount of dianhydride employed in the complexing step can vary overa wide range depending upon the fractionation desired. The amount ofdianhydride used is related to the amount of DMN present. If anextremely large ratio of dianhydride to DMN is used and sufficient timeallowed, all the DMN would complex and no DMN fractionation could beobtained. On the other hand, the amount of dianhydride used can begreater than the amount necessary to ultimately form a complexatecontaining all the DMN in the eutectic being treated if the length oftime of contacting is relatively short. Preferably, the amount ofdianhydride contacting the dissolved eutectic is in the range from 0.01to 3.0 moles of dianhydride per mole of DMN. A more preferable range isfrom 0. 10 to L5 moles of dianhydride per mole of DMN.

A DMN eutectic is solid at room temperature, but for complexation tooccur substantially all the eutectic must be dissolved in an inertsolvent. The solvent must be one that will not react with thedianhydride but one in which the eutectic will dissolve. Inert solventssuch as C to C alkanes, alkenes, cycloalkanes, cycloalkenes or mixturesthereof, CC], and ethers are suitable. Of the C to C hydrocarbons, thealkanes are preferable because of their lower cost. Some examples ofsuitable ethers are: diethyl ether, diisopropyl ether, ethyl nbutylether, divinyl ether, di-n-hexyl ether.

In addition, there is some minimum DMN concentration in an inert solventat which complexation will not occur. For example, if a simple mixtureof decane and 2,6-DMN at ambient temperature is treated with a molarexcess of PMDA, no complex will form if the concentration of the 2,6-DMNis less than about 1 weight percent. Also, if a simple mixture of decaneand 2,7-DMN at ambient temperature is treated with a molar excess ofPMDA, no complex will form if the concentration of 2,7-DMN is less thanabout 3 weight percent. As discussed hereinafter, this minimumconcentration phenomenon can be used to release the DMN from the formedcomplex. As for other combinations of DMN and dianhydride, this minimumconcentration can be determined by the methods discussed hereinafter.

Complexing occurs over a relatively wide temperature range with the rateof complex formation increasing with increasing temperature. The lowertemperature limits are dictated by practical considerations regardingthe rate of complex formation. The upper temperature limits of theprocess are governed by the thermal stability of a given complex. Thusit is apparent that the optimum temperature for the operation of thepresent process depends upon both the rate of complex formation and thestability factor.

In general, the highest temperature employed will be below the meltingpoint of the dianhydride, i.e., 540-546 F. for PMDA and 382386 F. forDA.

In general, the lowest temperature employed will be above the freezingor melting point of the inert solvent. For example, pentane has afreezing point of about 200 F. and eicosane has a melting point of about97 F., cyclohexane 45 F., l-hexane 220 F. and cyclohexene 153 F.

The preferred temperature range for the complexing step is between lO-375 F., more preferably between I0O-350 F.

The solid complex is readily separated from the solvent- DMN-anhydridemixture. Filtration, decantation or centrifugation can be used to removethe complex. Separation of the complex from the mixture is ordinarilyperformed at a temperature below about 150 F.; temperatures between 50and 100 F. are particularly effective for dianhydride-DMN complexseparation. Lower separation temperatures, e.g., 0 F., can also be used.

DMN can easily be separated from the complex by heating the latter undervacuum and recovering the DMN as a distillate. By employing such apreferred operation, the dianhydride is regenerated and can be reusedfor further complexing. Recovery of the DMN can also be done by elutionof the complex with an inert solvent such as decane or destruction ofthe dianhydride by such agents as water or an aqueous base.

If an inert solvent is used to elute the complex, a sufficient quantitymust be used. The quantity necessary can depend on the particular DMN inthe complex and the temperature used. Thus, for example, if a2,6-DMN-PMDA complex is eluted at an ambient temperature with a C to Calkane, such as decane, the amount of decane used must be such that theresulting concentration of 2,6-DMN in the 2,6-DMN-decane mixture is lessthan one weight percent. However, if the temperature of elution ishigher than ambient temperature, the concentration of 2,6-DMN in theresulting 2,6-DMN-decane mixture can be greater than one weight percent.

Thus in this invention a DMN eutectic is placed in a suitable inertsolvent, preferably C to C alkane, and dissolved. The resulting solutionis then contacted in the liquid phase with solid dianhydride.Alternatively, the solid dianhydride is placed in said solvent and thenthe eutectic dissolved. The

amount of dianhydride used is sufficient to preferentially com plex withone of the DMN. Generally, the temperature of contacting is lower thanthe melting point of the dianhydride. Furthermore, the temperature ofthe resulting mixture of DMN, solvent and dianhydride can be maintainedat ambient temperature, e.g., S0-IO0 F., until the desired or finalcomplexation is reached. Alternatively, the temperature of the resultingmixture, after contacting at the ambient temperature, can be elevated toa higher temperature, the latter being less than the melting point ofthe dianhydride to reduce the time required to reach the desiredcomplexation. The DMN- dianhydride complex can be separated from theresulting mixture at a suitable elevated temperature although it ispreferred to separate the complex after the DMN-dianhydride-solventmixture is at ambient temperature, i.e., 50-l00 F. However, separationcan occur at lower temperatures, e.g., 0 F or even just above thefreezing or melting point of the solvent. Suitable agitation of theDMN-solvent-dianhydride mixture can occur during or after the additionof the dianhydride and/or during the heating and/or any cooling steps.

The solid DMN dianhydride complex can be decomposed in several ways. Forexample, after the solid DMN-dianhydride complex has been separated, thecomplex can be heated under vacuum and the DMN removed as distillate.Preferably, the complex should be washed to remove liquid on the surfaceof the solids. This liquid will have a composition equal to theuncomplexed material and its presence reduces the effectiveness ofseparation. Another way to decompose the complex is to add a suitableinert solvent, i.e., C to C alkane, alkene, cycloalkane, cycloalkene andmixtures thereof, as a result of which the complex will decompose. Thesolvent should have a boiling point such that it can easily be separatedfrom the DMN by distillation. The solid dianhydride is removed and theremaining, for example, DMN-alkane mixture fractionated.

An alternative procedure comprises adding a suitable inert solvent andraising the temperature of the resulting complexsolvent combination.Upon decomposition of the complex, the dianhydride is removed from thehot inert solvent and the remaining DMN-solvent mixture fractionated. Inthis latter technique, the use of elevated temperatures reduces thenecessary amount of solvent.

Still another way to decompose the complex is to contact the separatedDMN-dianhydride complex with a compound which will react with thedianhydride, thereby releasing the DMN. Among such materials are water,aqueous sodium hydroxide, aqueous calcium hydroxide, etc. The advantageof using aqueous sodium hydroxide, etc., is that the formation of a saltwhich dissolves in the water enhances the separation of the complexatefrom the water. When the dianhydride is not reacted with any compound torelease the DMN, it can be used to contact untreated eutectic.

After complexation, the complexate and noncomplexate have compositionsdifferent from the starting eutectic thereby enabling either or bothproducts to be treated to obtain pure DMN. For example, if a eutectic of2,6-DMN and 2,7-DMN has been treated, the complexate can be treatedfurther to isolate pure 2,6-DMN. Also, the noncomplexate material isenriched in 2,7-DMN, thereby permitting pure 2,7-DMN to be isolated fromthe noncomplexed fraction. Any eutectic of 2,6- DMN and 2,7-DMN formedby this additional processing can be complexed again with dianhydrideand, subsequently, pure 2,6-DMN and pure 2,7-DMN can be isolated fromthe second eutectic.

The following examples illustrate this invention:

EXAMPLES I-IIl Three runs were performed to show that PMDA willpreferentially complex with 2,6-DMN in a eutectic mixture of 2,6-DMN and2,7-DMN at various ratios of moles of PMDA per mole of DMN. The resultsof these runs are shown in the following Table II. In all runs theconcentrations of 2,6-DMN in the complexate is substantially greaterthan that in the eutectic.

TABLE II PMDA Complexation with Eutectic 2,6- & 2,7-DMN Mixture EutecticDMN mixture contains 41.5 weight percent 2,6-DMN and 5815 weight percent2.7-DMN In these three runs grams of the solid eutectic containing 41.5weight percent 2,6-DMN and 58.5 weight percent of 2,7- DMN weredissolved in 50 grams of decane. To the resulting solution, a suitableamount of PMDA was added. The resulting mixture was slowly heated toabout 275 F. and then cooled to ambient temperature and allowed toremain at this lower temperature for about 30 minutes. Then theresulting mixture was filtered and the separated solid complex waswashed with hexane and vacuum dried. The solid complex was placed in asufficient amount of decane to decompose the complex and the mixture washeated to 250 F. The solid PMDA was filtered from the hot decane andcomplexate. The remaining liquid was stripped of decane to yield thecomplexate. Analyses of the complexate and noncomplexate are shown inthe preceding Table II.

The PMDA used was a white powder with a purity of 98* percent. ltsmelting point was 540560 F.; particle size was 95 percent less than 10microns; its boiling point was 74575 2 F. and its specific gravity was1.68.

The aforementioned eutectic can be fractionated in an analogous mannerusing DA. Similarly, the other binary eutectics can be fractionated inan analogous manner using either dianhydride, i.e., PMDA or DA.

The invention claimed is:

l. A method of fractionating a binary eutectic consisting essentiallyofdimethylnaphthalenes having each methyl group at a B positioncomprising:

a. dissolving said eutectic in an inert solvent and contacting resultingsolution with a solid complexing dianhydride ofl,2,3,4-benzenetetracarboxy1ic acid or of 1,2,4,5-benzenetetracarboxylic acid at a temperature within the range from themelting point of the dianhydride to the freezing point of said solvent,to complex preferentially with one of the dimethylnaphthalenes and forma solid complex containing less than the total amount ofdimethylnaphthalenes in said eutectic;

b. separating the solid complex from the resulting admixture;

. and decomposing the solid complex to recover the resulting complexatehaving a proportion of dimethylnaphthalenes different from that in thestarting eutectic.

2. A method according to claim 1 wherein the solution of eutectic andsolvent is contacted with the dianhydride at 50100 F., the temperatureof said resulting admixture is increased to within the range from l00375F. and the solid complex is separated at a temperature between thefreezing point of the solvent and F.

3. A method according to claim 1 wherein the temperature at which thedianhydride is contacted with the solution of eutectic and solvent is inthe range from 100-350 F. and the solid complex is separated at 0100 F.

4. A method according to claim 1 wherein the amount of dianhydridecontacting the eutectic is in the range from 0.01 to 3.0 mole per moleof dimethylnaphthalene.

5. A method according to claim 1 wherein the eutectic consists of2,6-dimethylnaphthalene and 2,7-dimethyl naphthalene, the inert solventis alkane of the C to C range,

the dianhydride is that of 1,2,4,5-benzenetetracarboxylic acid,

the amount of said dianhydride contacting the mixture is in the rangefrom 0.1 to 1.5 mole per mole of dimethylnaphthalenes, the temperatureat which said dianhydride is contacted with the solution of eutectic andsolvent is in the range from 100350 F. and the solid complex isseparated at 6. A method according to claim 1 wherein the dianhydride isthat of l,2,4,S-benzenetetracarboxylic acid.

7. A method according to claim 6 wherein said eutectic is composed of2,6-dimethy1naphthalene and 2,7-dimethylnaphthalene.

8. A method according to claim 1 wherein the dianhydride is that of l,2,3,4-benzenetetracarboxylic acid.

9. A method according to claim 8 wherein said eutectic is composed of2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene.

2. A method according to claim 1 wherein the solution of eutectic andsolvent is contacted with the dianhydride at 50*-100* F., thetemperature of said resulting admixture is increased to within the rangefrom 100*-375* F. and the solid complex is separated at a temperaturebetween the freezing point of the solvent and 100* F.
 3. A methodaccording to claim 1 wherein the temperature at which the dianhydride iscontacted with the solution of eutectic and solvent is in the range from100*-350* F. and the solid complex is separated at 0*-100* F.
 4. Amethod according to claim 1 wherein the amount of dianhydride contactingthe eutectic is in the range from 0.01 to 3.0 mole per mole ofdimethylnaphthalene.
 5. A method according to claim 1 wherein theeutectic consists of 2,6-dimethylnaphthalene and2,7-dimethylnaphthalene, the inert solvent is alkane of the C5 to C20range, the dianhydride is that of 1,2,4,5-benzenetetracarboxylic acid,the amount of said dianhydride contacting the mixture is in the rangefrom 0.1 to 1.5 mole per mole of dimethylnaphthalenes, the temperatureat which said dianhydride is contacted with the solution of eutectic andsolvent is in the range from 100*-350* F. and the solid complex isseparated at 0*-100* F.
 6. A method according to claim 1 wherein thedianhydride is that of 1,2,4,5-benzenetetracarboxylic acid.
 7. A methodaccording to claim 6 wherein said eutectic is composed of2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene.
 8. A methodaccording to claim 1 wherein the dianhydride is that of1,2,3,4-benzenetetracarboxylic acid.
 9. A method according to claim 8wherein said eutectic is composed of 2,6-dimethylnaphthalene and2,7-dimethylnaphthalene.